Vertex parent disappears when joining meshes?
If you've the possibility to place the button close to (or just on) a mesh vertex, you can report the bones influences of this vertex to the button.
Here is a magnificent example with a very beautiful dress and no less beautiful buttons:

To do that, once your dress is rigged (and again the button can be placed where a dress vertex is), select the button vertices, and shift select (as active) the dress vertex:

Vertical green stroke indicates the weights of the dress vertex (active).
Use the copy button to report these weights.
The button may deform a bit, but as you want all in one mesh, I think you can't avoid it.
A test case: